#include <iostream>


using namespace std;





int n,m,i,j,a,b,head,tail,ans;


int graph[100][100];


int degree[100];


int len[100];


int queue[100];


int main() {


    cin>>n>>m;


    for(i=0; i<n; i++)


        for(j=0; j<n; j++)


            graph[i][j]=0;


    for(i=0; i<n; i++)


        degree[i]=0;


    for(i=0; i<m; i++) {


        cin>>a>> b;


        graph[a][b]=1;


        (degree[b]++);


    }


    tail = 0;


    for (int i=0; i<n; i++)


        if(!degree[i]) {


            queue[tail]=i;


            tail++;


        }


    head= 0;


    while (tail<n) {


        for (i=0; i<n;  i++)


            if (graph[queue[head]][i]== 1) {


                (degree[i]--);


                if (degree[i]==0) {


                    queue[tail] = i;


                    tail++;


                }


            }


        (++head);


    }


    ans = 0;


    for (i=0; i<n; i++) {


        a =queue[i];


        


        len[a]=1;


        for (j=0; j<n; j++)


            if(graph[j][a]==1&& len[j]+1>len[a])


                len[a]=len[j]+1;


        if((len[a]> ans))


            ans = len[a];


    }


    cout<<ans<<endl;


    return 0;


}

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